A number of important endogenous small ligands (e.g., tryptophan) as well as many drugs (e.g., phenytoin, salicylates) are less tightly bound to plasma proteins of uremic patients than normal subjects. In addition, the micro-forms of plasma albumins are abnormal in uremia. We have recently isolated and partially purified a dialyzable solute(s) from uremic body fluids which, when added to normal plasma, reproduces the abnormal binding of uremic plasma. The inhibitor(s) will be purified to homogeneity by hydrophobic and ion-exchange chromatography, gel filtration and electrophoresis. Its structure will be determined using infrared spectroscopy, functional group determinations, derivative formation and comparison with pure known compounds by chromatography. Methods to directly measure the binding inhibitor in uremic plasma will be tested. Albumin will be isolated from normal and uremic plasma by affinity chromatography. Amino acid composition and the pattern of micro-formes on isoelectric focusing will be determined. The degree of abnormality in ligand binding will be correlated with the pattern of micro-forms of albumin over a wide range of renal function in patients. Finally, the effect of the pure inhibitor on binding parameters of a number of drugs and natural substances will be determined.